Various unit-magnification optical imaging systems are known in the patent literature. Patents related to unit-magnification optical system comprising a concave spherical mirror and a convex spherical mirror include U.S. Pat. No. 3,748,015, U.S. Pat. No. 4,293,186, U.S. Pat. No. 4,711,535, and U.S. Pat. No. 4,796,984.
U.S. Pat. No. 3,748,015 describes a unit-magnification imaging catoptric system comprising a concave spherical mirror and a convex spherical mirror arranged with centers of curvature thereof coincident. There is an aperture stop at the convex mirror. The concentric mirrors are arranged to produce at least three reflections within the system. Two off-axis conjugate areas at unit magnification are coplanar in this system. The axis of this system lies normal to the coplanar object and image planes and through the common centers of curvature of the mirrors. Like most prior-art unit magnification projection systems, embodiments described in '015 patent are symmetric relative to the aperture stop, i.e., are systems consisting of two identical subsystems disposed symmetrically about the (central) aperture stop. Such a symmetric, imaging, catoptric system is intrinsically free of coma and distortion. Since the mirrors disclosed in the '015 patent are concentric, this imaging system is also free of spherical aberration. This optical system is a narrow ring-field design providing sharp imagery only over a quite narrow annular area in the focal plane. In photolithography, such a system is used with a narrow slit aperture to expose this narrow area, and to copy an object (mask) to an image surface by scanning the object and image across this aperture, in synchronism.
U.S. Pat. No. 4,293,186 describes a unit-magnification catadioptric optical imaging system which is an improvement of the catoptric system described in U.S. Pat. No. 3,748,015. U.S. Pat. No. 4,293,186 discloses a system having refractive elements, in addition to reflective elements. This system has means for obtaining stigmatic imagery, in a restricted off-axis field, over an extended spectral range, by balancing the chromatic variation in focus at the center of the restricted off-axis field, due to variation of field curvature, with color by introducing axial color aberration of the opposite sense.
U.S. Pat. No. 4,711,535, discloses another unit-magnification, restricted off-axis, ring-field, catadioptric optical imaging system having broad spectral range and providing improvements to the catoptric system described in the '015 patent, by having optical elements arranged and constructed such that the sum of the refractive powers is nearly zero, and the sum of the reflective powers is also nearly equal to zero. This system includes convex and concave spherical mirrors, pairs of nearly concentric meniscus lens elements, and a pair of identical thick flat parallel plates located adjacent to the object and image planes. The thick flat parallel plates are used to cancel the chromatic aberrations introduced by the meniscus elements.
U.S. Pat. No. 4,796,984 discloses a substantially unit-magnification catadioptric optical imaging system, comprising at least one convex mirror, and at least one concave mirror. The mirrors are supported with their centers of curvature substantially coincident, and means are provided to define a location for an object, the image of which, after at least three reflections including at least one reflection at each of the mirrors, is a real image at a second location. This system further comprises a monocentric meniscus lens between the concave and convex mirrors, and gives overall correction of the Petzval sum for the system to produce a stigmatic image.
It is well known in the optics literature that meniscus lens elements can be used to reduce or correct spherical aberration of principal rays parallel to the optical axis. The application of meniscus lenses for correcting the spherical aberration of the principal rays was described in a book by A. Bouwer, entitled “Achievements in Optics,” pages 24, 25, and 39, Elsevier Publishing Company, Inc., 1946. Another publication related to the use of meniscus lens element is a paper by D. D. Maksutov, entitled “New Catadioptric Meniscus System,” J. Opt. Soc. Am. 34(5), pp. 270-284 (1944). An additional publication describing unit magnification imaging systems with compensation meniscus lenses appears in the Soviet Journal of Optical Technology, 50(3), March 1983, p. 153.
The use of concentric optical elements is also well known in the optics literature. Publications related to the use of concentric optical elements include the paper by J. Dyson, entitled “Unit magnification optical system without Seidel aberrations,” J. Opt. Soc. Am. 49(7), pp. 713-716 (1959) and a paper by C. G. Wynne in the articles “A unit power telescope for projection copying,” Optical Instruments and Techniques, Oriel Press, Newcastle upon Tyne, England (1969), and “Monocentric telescope for microlithography,” Opt. Eng. 26(4) 300-303 (1987).
The unit-magnification imaging optical systems described in the above-cited references give sharp imagery only over narrow annular area in the focal plane. While the projection lens designs described in these cited patents are quite suitable for normal photolithography applications at 404 nanometers (nm), 365 nm and 248 nm wavelengths, such lens designs have not provided adequate capabilities when the object and image surfaces are separated to more convenient accessible locations by the insertion of plane fold-mirrors, as is required for other applications such as exposure equipment using an illumination source at a laser diode wavelength, for example, 808 nm, 980 nm, or 1024 nm, and requiring large rectangular field sizes, large working distances, and compact packaging volume. The design embodiments described in these above-referenced patents are not suitable to be packaged in a compact volume enclosure for exposure systems requiring large rectangular exposure fields with lengths ranging from one-hundred to a few hundred millimeters (mm) and working distances of at least 100 mm from the system package envelope enclosure. Such distances and dimensions are required for masked laser-patterning apparatus in the manufacture of liquid crystal, LED, and OLED display panels or screens. Due to these shortcomings of the prior art, it is desirable to develop optical designs of large-field unit-magnification projection optical systems capable of imaging, in one exposure, large rectangular object fields with lengths greater than 100 mm, and having working distances greater than 100 mm to significantly increase system throughput in masked laser-patterning apparatus.